1. Field of the Invention
The present invention relates to a vertical wafer boat which supports a number of semiconductor wafers in order to set the wafers into a furnace during a process such as oxidation of the semiconductor wafers, diffusion, or vapor phase growth, and particularly to a vertical wafer boat which is suitably used for forming a SiO.sub.2 film, a Si.sub.3 N.sub.4 film, a poly-Si film, or the like on the surfaces of semiconductor wafers by a method such as the LP (Low Pressure)-CVD(Chemical Vapor Deposition) method.
2. Description of the Related Art
Conventionally, as a vertical wafer boat of this kind, known is a wafer boat disclosed in Japanese patent unexamined publication (JP-A HEI-6-183676) applied by the applicant the present application.
In the wafer boat, as shown in FIGS. 15 and 16, four support rods 34 in each of which a number of wafer mount groove portions 33 are formed in one edge and which have a triangular section shape are disposed between upper and lower semi-arcuate plates 31 and 32 so that the wafer mount groove portions 33 are respectively directed to the centers of semiconductor wafers (not shown) to be mounted on the wafer mount groove portions. The plates 31 and 32, and the support rods 34 are made of SiC or SiC infiltrated by Si (Si--SiC). It is also well known that a surface of a wafer boat is covered by a high purity ceramic film in order to obtain the wafer boat having a high purity.
In the wafer boat configured as described above, since the support rods 34 have a triangular section shape, the buckling load is increased and the useful life is extended.
FIGS. 17 to 19 show another vertical wafer boat of the prior art. In the wafer boat, between upper and lower ring plate-like plates 36 and 37 in each of which a slit 35 elongating in the wafer insertion direction (in FIG. 18, from the right side to the left side) is formed in the center of the rear portion (in FIG. 18, the left side), four support rods 39 in which a number of wafer mount groove portions 38 are formed and which have a circular section shape are disposed so that the wafer mount groove portions 38 are respectively directed to the centers of semiconductor wafers "W" to be mounted on the wafer mount groove portions. The plates 36 and 37, and the support rods 39 are made of a ceramic base material and the outer surfaces of the vertical wafer boat constituted by them are covered by a high-purity ceramic film (not shown). In the support rods 39, a chamfer 40 is formed in the lower horizontal edge of each of the wafer mount groove portions 38. A projection 42 protrudes from the front portion (in FIG. 18, the right side) of the lower plate 37. A reference face 41 which serves as the reference for automatic transfer of the semiconductor wafers "W" is formed on the lower face of the projection.
In the vertical wafer boat, when a semiconductor wafer "W" is to be transferred, the position of the corresponding wafer mount groove portion 38 is obtained on the basis of the reference face 41, thereby enabling the semiconductor wafer "W" to be automatically transferred.
In the former prior art vertical wafer boat, since the support rods have a triangular section shape, the ceramic film on the vertical edges are easily broken by a contact with another member during production or use, or concentration of thermal stress due to heat cycles during use. In such a case, there arises a problem in that impurities may be emitted from the ceramic base material and the semiconductor wafers to be treated may be adversely affected by the impurities.
When an SiO.sub.2 film, an Si.sub.3 N.sub.4 film, a poly-Si film, or the like is to be formed on the surfaces of semiconductor wafers by a method such as the LP-CVD method, the CVD film is formed not only on the semiconductor wafers but also on the vertical wafer boat itself. Therefore, this causes a problem in that, during a subsequent wafer transfer process or heat processing, the CVD film deposited on the vertical edges of the support rods and the upper and lower horizontal edges of the wafer mount groove portions may be peeled off to function as particles and the semiconductor wafers to be treated may be adversely affected by the particles.
In the latter prior art vertical wafer boat, since the support rods have a circular section shape, the problems produced in the former wafer boat do not occur. However, the horizontal sectional view of the wafer mount groove portions form an acute angle, and hence there arises a problem in that the ceramic film formed on the edges and the CVD film deposited on the edges may be peeled off.
The vertical wafer boats may undergo a cleaning process after each step. Usually, however, such a cleaning process is not performed, because the productivity is impaired and a part of the ceramic film may not be cleaned away.
Since both the vertical wafer boats have the four support rods, the number of portions where semiconductor wafers are supported is large. This produces a problem in that a large number of particles may be produced.